1. Field of the Invention
The present invention relates to the design of a semiconductor package; and, in particular, the present invention relates to a semiconductor package designed for high electrical and thermal dissipation performances.
2. Discussion of the Related Art
Semiconductor devices are becoming larger, integrating a larger number of circuits, and operating at increasingly higher clock frequencies. As a result, semiconductor devices are requiring, without compromising reliability, packages of increasingly higher lead count, and higher electrical and thermal performances.
In the prior art, conventional plastic molded packages can dissipate up to 2 watts of power. With some improvements in the lead frame, and by adding a heat spreader or heat sink, a plastic molded package can dissipate up to 4 watts. A further improvement in power dissipation can be achieved by attaching the semiconductor device, also called the semiconductor xe2x80x9cdiexe2x80x9d, onto an integral heat sink. Such a heat sink typically has a surface exposed to the ambient to conduct heat away from the package. An example of such a package, also called a xe2x80x9cthermally enhancedxe2x80x9d package, is shown in FIG. 1.
FIG. 1 shows a thermally enhanced package comprising a semiconductor die 105 attached by a layer of thermally conductive epoxy to a metallic heat sink 101. The input and output terminals of semiconductor die 105 are electrically coupled to connection terminals (xe2x80x9cleadsxe2x80x9d) of a lead frame 103 by wire bonds 104, which connect the bonding pads of semiconductor die 105 to individual leads in lead frame 103. Lead frame 103 attaches to heat sink 101 by a layer of dielectric adhesive 107. The thermally enhanced package is encapsulated in a plastic molding 102. In package 100, high thermal dissipation is achieved by attaching semiconductor die 105 directly onto the lower surface of heat sink 101 using a thermally conductive epoxy layer 106.
Although plastic molded packages are typically of high reliability, the incorporation of a heat sink in a thermally enhance plastic molded package, such as package 100 of FIG. 1, leads to failures which are directly related to the design and the material used in the heat sink. For example, heat sink 101 is often made of aluminum. The large difference between the thermal expansion coefficients (TCE) of the silicon die, at 3 ppm/xc2x0 C., and of aluminum, at 25 ppm/xc2x0 C. induces significant strain on semiconductor die 105. Such strain causes die- cracking and thus a package failure. For this reason, in the prior art, semiconductor die sizes are kept well below 10xc3x9710 mm to minimize the induced stress. Alternatively, a heat sink material with lower TCE can be chosen to minimize the large mismatch in the heat sink""s and the semiconductor die""s coefficients of thermal expansion.
A similar mismatch in TCEs exists between heat sink 101 and the plastic molding 102. Typically, a plastic molding compound has a TCE of about 17 ppm/xc2x0 C. The thermal cycle package 100 experiences during assembly and normal operations induces high stress at the metal-to-molding interface (i.e. between heat sink 101 and plastic molding 102) which can lead to delamination, cracking of the molding, and die failures. For this reason, a close matching of the TCEs of heat sink 101 to plastic molding 102 is very desirable.
During the assembly of package 100, plastic molding 102 shrinks significantly after the molding operation and during post-mold curing, which is typically carried out at or about 175xc2x0 C. The shrinking molding causes significant stress at the metal-to-molding interface, which can lead to delamination. Delamination is very undesirable and usually causes long-term reliability failures. Delamination can be minimized by including on the heat sink xe2x80x9clockingxe2x80x9d features, which strengthen mold adhesion, and by choosing a heat sink material with a TCE closer to that of the molding compound.
In the prior art, frequency performance is limited by the electrical parasitic impedances of the lead frame to 50 MHz or less. The lead frame usually consists of a single metal layer without the ability to provide controlled impedance connections. U.S. Pat. No. 4,891,687, entitled xe2x80x9cMultilayer Molded Plastic IC Packagexe2x80x9d, to Mallik et al, filed on January 27, and issued on Jan. 2, 1990, discloses a package achieving a high electrical performance. However, the package disclosed in U.S. Pat. No. 4,891,687 requires two lead frames, and hence, such package is significantly more costly than a conventional plastic molded package.
Furthermore, the prior art""s use of long wire bonds between the semiconductor die and the lead frame increases the impedances of ground connections. A high impedance to a ground connection results in xe2x80x9cground bouncexe2x80x9dand other electrical noises which further restrict the overall electrical performance of the conventional plastic molded package. In logic semiconductor devices, which usually require high lead counts, about 25% of the leads in each package are used for power and ground connections. The large number of leads devoted to power and ground connections significantly reduces the number of pins available for signal connections, which usually determine the level of available performance.
In accordance with the present invention, a plastic molded package is provided comprising (i) a heat sink having an upper surface and a lower surface, (ii) a ceramic or dielectric ring attached by an adhesive film to the lower surface of the heat sink; (iii) a semiconductor die attached using a thermally conductive epoxy adhesive to the lower surface of the heat sink through an aperture in the dielectric ring; (iv) a lead frame, which is attached to a surface of the dielectric ring, having a number of leads extending outside of the plastic molded package; and (v) a plastic molding enclosing the ceramic ring, the lead frame, except at the exposed portion of the leads and the semiconductor die.
In accordance with one aspect of the invention, the heat sink comprises a base portion enclosed in the encapsulation and a raised portion protruding above the base portion having a surface exposed to the ambient. In one embodiment, the exposed surface of the raised portion is free of corners (e.g. in the shape of a circle). The base portion of the heat sink includes a number of conical protrusions enclosed in the molding, and a number of through holes filled by the molding. The exposed portion of the raised surface of the heat sink is coated with nickel to provide a good conductive surface for attaching an external heat sink. Suitable materials for the heat sink includes oxygen free high conductivity copper, copper/molybdenum/copper laminate, copper/tungsten/copper laminate and beryllium composites.
In accordance with another aspect of the present invention, the lead frame of the plastic molded package further comprises an interposer ring downset and attached to the heat sink. The interposer ring comprises either a single loop (360xc2x0), or a number of electrically isolated sections for independent connections to power and ground terminals. Such electrically isolated sections of the interposer ring can be supported in the encapsulation by tie bars of the lead frame. For an electrically isolated section of the interposer ring, an electrical short to the heat sink allows the heat sink to be used as a ground plane for the semiconductor die. That electrical short can be accomplished by a drop of electrically conductive adhesive. The leads of the lead frame allow the internal power and ground planes in the interposer ring to be connected to power and ground supplies outside of the plastic molded package.
In accordance with another aspect of the present invention, the dielectric ring comprises a material selected from the group consisting ceramic materials, epoxy materials including Ablefilm 564 AKHM, and a dielectric sheet material sold under the trade name of Neoflex. A ceramic dielectric ring provides higher thermal conductivity than the other materials. Thus, the heat from the semiconductor die can be conducted through the wire bonds to the heat sink, rather than through the leads to the ambient, which is a path of much higher thermal impedance. Consequently, the package of the present invention provides higher performance in power dissipation.
In a package of present invention, an 8-watt thermal performance is achieved by adding the combination of the ceramic dielectric ring and an integral heat sink made out of oxygen-free high-conductivity copper (OFHC). Since copper""s TCE is 17 ppm/xc2x0 C., which is significantly less than aluminum""s TCE, the package of the present invention can attach a larger die than the prior art (e.g. up to 14xc3x9714 mm) without the risk of a die-cracking failure. Furthermore, in the present invention, the combined effects of the unique locking features on the integral heat sink, and the close matching of TCEs between copper and the molding compound, eliminate delamination and cracking failures modes observed in aluminum heat sinks of the prior art.
Further, by providing controlled impedance traces and separate power and ground rings on the lead frame, electrical performance in the packages of the present invention is significantly enhanced over the prior art. Controlled impedance is achieved by connecting the heat sink to electrical ground and using the heat sink as an electrical ground plane. By attaching a ceramic ring of an appropriate thickness as a dielectric layer between the heat sink and the lead frame, an impedance in the range of 40-60 ohms is achieved. Such controlled impedance provides high frequency performance in the range of 100 MHz.
The power and ground rings in a package of the present invention are provided separately to allow low impedance connections between the semiconductor die and the leads. Such low impedance connections are achieved by retaining only a peripheral part of a conventional die attach pad either as an entire ring or divided into two or more sections. In one embodiment, the entire ground ring is electrically shorted to the heat sink. Alternatively, in a second configuration, the two or more sections of the retained peripheral part of the die attach pad are each shorted to either a ground plane (e.g. the heat sink) or one or more power terminals. Such a configuration has lower inductance than the prior art leads because the wire bonds to the rings are shorter, and because the rings have a larger width than the leads. Furthermore, since most of the power and the ground connections are internal to the package, the available lead count for signal transmission is significantly increased. The higher lead count allows higher performance and achieves a cost which compares favorably with a conventional package of comparable performance.
The present invention is better understood upon consideration of the detailed description below and the accompanying drawings.